2. As to the _cause_ of rapid changes of form during revolutionary or critical periods in the earth’s history, Brooks has introduced an idea which is very suggestive, and deserves serious attention. We have above spoken of the progressive element as external. Brooks regards both elements as internal, and represented by the two sexes. The male represents the progressive, the female the conservative element. The one tends to divergent variation, the other to fixity of type by heredity. I think we will all admit that, as a general rule, in man (and probably all the higher animals) the male is more highly differentiated into many divergent forms--the female is more like the type-form of the species. In man, the male is certainly more diversified in form, in expression, and in character. If they have the keenest ear for musical pitch, they are also most often music-deaf; if they have the sharpest perception of color, they are also most often color-blind; if among them we find the brightest intellects, we also find the dullest and most stupid; if there are among them more geniuses, so, also, there are more cranks. The same is also, probably, true of other animals, in proportion to their grade of organization. The operation of these two equally necessary elements is well shown in every advancing society. The initiative of every movement, in all directions, good or bad, is determined by the male; the conservation of whatever balance of good there may be, seems to be mainly by the female. The male tries all things, the female holds fast that which is good. By the one society gains a little in each generation; by the other the gain is conserved and made a new point of departure. The one is ever building hastily a scaffolding and platform; the other ever consolidating into a permanent structure. Now, according to Brooks, what is true in the plane of social progress is true also in the lower plane of organic evolution. In sexual union, and in the resulting offspring, the sperm-cell is the element which tends to divergent variation, and the germ-cell to fixity of type, through heredity. In artificial breeding, then, we ought to make new varieties by proper use of the sire; we ought to preserve them true by proper management of the dam. But, again, it is believed that in many lower animals, especially insects, the high-feeding of the mother, and consequent good condition of the ovum, tends to the production of female offspring. It seems almost certain that, in butterflies, the sex is not yet declared in the caterpillar stage. According to the careful experiments of Mrs. Treat,[37] if the caterpillars be well fed, they become female butterflies; but, if poorly fed, they make males. One purpose of this provision of Nature is, doubtless, to provide for the greater draught on the vitality of the female in reproduction.